Our investigation of human myelin basic protein (HBP) extracted from whole brain has led to the isolation and identification of a new form of HBP with a molecular weight of 17.2 kDa. The more common form is an 18.5 kDa protein. The new form of HBP was successfully separated from HBP-component 3 by a newly developed procedure on FPLC (Fast Protein Liquid Chromotography). Based on recoveries from each step of the procedure we estimated that the new form constituted about 13% of the total HBP. We have shown that both the 17.2 and the 18.5 kDa proteins have blocked amino termini and identical carboxyl termini. When the HPLC elution patterns of the two proteins were compared, we found that four peaks in the chromatogram of the larger protein were missing from the chromatogram of the 17.2 kDa protein. In addition an extra peak was found in the elution pattern of the latter. Amino acid analyses and UV spectra of the individual tryptic peptides indicated that the smaller protein lacked residues 106-116 (Gly-Arg-Gly-Leu-Ser-Leu-Ser-Arg-Phe-Ser-Trp). The deleted portion corresponds exactly to the amino acid sequence encoded by Exon 5 of the mouse basic protein gene. When our study was essentially complete we discussed the data with Dr. Kamholz who had been working on the human myelin basic protein gene. Based on our data, he synthesized a cDNA probe for this new human BP form and isolated the corresponding mRNA, thus confirming our discovery. We are completing the investigation of the effect of the phosphorylation of residue 98 on the conformation of bovine myelin basic protein (BBP). Preliminary circular dichroic studies on heterogeneously phosphorylated BBP showed that the per cent of ordered structure (Beta-turn plus Beta structure) increased from 20% to 46% with larger amounts of phosphate on the BBP molecule.